The present invention relates in general to the forming of metal workpieces by cold deformation.
Among the various cold-forming processes, mention may first of all be made of metal extrusion or cold forging, which corresponds to a forming process consisting in making the mass of metal flow under a compressive force between a punch and a die. In this way it is possible to obtain various workpieces of well-defined geometrical shape. This type of deformation requires vertical or horizontal presses comprising one or more work stations equipped with transfers or not.
Another cold-deformation technique, similar to extrusion, is known as cold pressing. In this case, one or more deformation steps are carried out in a single machine, generally horizontal machines comprising one or more stations. These workstations are generally fed with a metal wire which undergoes plastic deformation under forces which are generally lower than in the case of actual extrusion.
Finally, mention may be made, by way of example of a cold-forming process, of wire drawing, which in fact constitutes an intermediate or preliminary preforming step, starting from a reel of wire in order to obtain lengths of smaller diameter, generally intended to feed a cold-pressing station. This type of deformation is used mainly upstream of the manufacture of screws and bolts.
This cold-forming technology is applicable to a very large number of steels and generally nonferrous alloys. In general, the operations are carried out at ambient temperature starting with slugs, blanks or preforms that have undergone a specific preparation operation.
By way of example of types of cold deformation possible, mention may be made of flattening, preforming, forward extrusion or reverse extrusion, hollow or xe2x80x9cenfiladexe2x80x9d forward extrusion, lateral extrusion, stretching, upsetting, sizing or even cone forming.
Extrudable steels capable of undergoing such cold deformations fall within various categories, especially general-purpose nonalloy steels, but preferably special nonalloy steels for heat treatments, generally fine carbon steels, special alloy steels for heat treatments, stainless steels or microalloy steels. The latter can be cold formed without annealing and acquire, by cold working, high mechanical strength levels while still retaining an acceptable residual ductility.
One of the main difficulties to solve in the context of this technology of the cold deformation of metal workpieces is the need to have to carry out, before forming, surface pretreatments usually involving successive operations which are quite lengthy and expensive, and sometimes relatively difficult to implement, and the effectiveness of which is not entirely satisfactory.
The quality of the surface treatments, for example treatments specific to extrusion, determines the good result obtained after the deformation operations. The essential aim of these surface treatments before forming is, of course, to reduce as far as possible the friction forces exerted in the tooling.
It is precisely the forces involved in cold-forming operations of this type which constitute the major obstacle to the development of these extrusion techniques.
It is therefore essential to be able to reduce the friction forces so as to prevent seizure of the workpiece as far as possible, to reduce the load needed for extrusion and to minimize the wear of the tooling.
These pretreatment operations, mainly based on the lubrication of slugs or preforms, may have to be carried out between two successive deformation operations, whether or not the workpieces undergo an annealing operation.
In the case of carbon steels or low-alloy steels, the pretreatment firstly involves alkaline cleaning and pickling in sulfuric acid in the presence of an inhibitor, the purpose of which is to limit the attack of the metal itself, followed by phosphatizing and finally the actual lubrication.
The purpose of the phosphatizing operation is to form a first, generally porous, adhesion layer of zinc phosphate which is intended to receive the lubricant. Deposition of this lubricant, generally consisting of zinc stearate resulting from the reaction of soaps that react with the zinc phosphate layer, is difficult to control in practice. This is because it is necessary to tailor the thickness of the zinc stearate layer according to the mechanical stresses to which the workpieces to be deformed will be subjected. This tailoring is all the more difficult to control as it involves controlling a chemical reaction which develops at depth within the thickness of the applied layers and the reaction time involved is several hours.
Consequently, the lubrication operation generally involves immersion of the prephosphatized material in hot baths of reactive soaps.
However, this combining between the zinc phosphate layer and the zinc stearate layer may remain insufficient to avoid any contact between the metal workpiece and the tooling.
If the zinc stearate layer is not satisfactory, other, more sophisticated, lubricating products must then be used, which require additional deposition operations, by immersing the workpieces or else by spraying, not only on the workpieces but also on the tooling. Such operations require constant monitoring of the concentration of the lubrication solution and of the application temperature in order to obtain coatings which unfortunately are generally quite irregular.
In the prior art, it has hitherto been regarded as indispensable to carry out a prior phosphatizing step in order to allow both good adhesion and the formation of zinc stearate fulfilling the function of lubricant for the workpiece to be deformed. In a number of applications, and mainly within the context of cold pressing, it is imperative, after forming of the workpiece, to dephosphatize the latter before carrying out the heat treatment in order to avoid any risk of phosphorus diffusing into the steel. Such heat treatments, generally carried out at temperatures of about 850 to 900xc2x0 C., are essential and actually result in modifications to the structure of the workpieces formed. Such a drawback of the prior art connected with the need to have to carry out a dephosphatizing operation before the heat treatment is particularly serious in the case of the production of screws and bolts, in which problems of embrittlement of the workpieces intended to be subjected to permanent stresses, often resulting in fatigue fractures, are observed.
It is an object of the present invention specifically to reduce, if not to completely eliminate, the abovementioned drawbacks. The subject of the invention is more particularly a process for forming metal workpieces by cold deformation, in a first operation of mechanically depositing a layer based on metallic zinc on the free surface of the workpiece to be produced, it being possible for this layer optionally to contain and/or to be coated with a layer of lubricant, in order subsequently to carry out the forming of said workpiece by plastic deformation of the metal.
Such a cold-forming process has allowed the plastic deformation phenomena to be greatly facilitated by reducing the friction forces involved, and possibly even going as far as reducing the number of intermediate steps during the forming procedure.
The process according to the invention makes it possible to overcome all the drawbacks connected with the use of a phosphatizing pretreatment of the metal blanks or slugs. Finally, it proves to be the case that certain workpieces produced by such a forming process, involving mechanical predeposition of a layer based on metallic zinc, make it possible to obtain workpieces whose fatigue lifetime is improved.
Depending on the forming process used, it may be sufficient to deposit on the metal slug only a single layer based on metallic zinc on the free surface of the latter. Such a layer, made of zinc or more generally of a zinc-iron alloy, or even a mixture of zinc and iron particles, may be applied within the context of the present invention in an amount of between 50 and 250 mg/dm2 of add-on metal. For certain special applications, smaller deposited amounts may even be satisfactory.
Such a layer may be sufficient itself to fulfill the lubrication function for cold-forming operations during which the metal is plastically deformed by relatively low forces.
Advantageously, the layer based on metallic zinc is mechanically deposited by a shot blasting operation with the aid of steel shot having at least one outer layer comprising either pure zinc or a zinc-based alloy.
Such mechanical deposition of a layer based on metallic zinc may also be carried out by a shot blasting operation with the aid of a mixture of steel shot and shot consisting of a steel core and having, on the surface, at least one outer layer based on a zinc alloy or an outer layer of pure zinc.
Finally, this mechanical deposition of the layer based on metallic zinc may also be obtained by shot blasting with the aid of shot based essentially on an iron alloy, the blasting being performed in the presence of a zinc powder or zinc semolina, which is therefore applied due to the mechanical effect of the blasting.
The term xe2x80x9cshotxe2x80x9d or xe2x80x9cmicroshotxe2x80x9d used within the context of the present invention to describe the shot blasting operations should be understood within the broad sense, that is to say they encompass all types of shapes of particles or microparticles to be blasted onto the surface of the workpieces.